News Release Number: STScI-2004-07

Hubble's Deepest View Ever of the Universe Unveils Earliest Galaxies

March 9, 2004: Astronomers at the Space Telescope Science Institute today unveiled
the deepest portrait of the visible universe ever achieved by
humankind. Called the Hubble Ultra Deep Field (HUDF), the
million-second-longREAD: Junior version of this articleLearn about this story in the Star Witness, a science newspaper available on our sister site, Amazing Space. exposure reveals the first galaxies to emerge
from the so-called "dark ages," the time shortly after the big bang
when the first stars reheated the cold, dark universe. The new
image should offer new insights into what types of objects
reheated the universe long ago.

This historic new view is actually two separate images taken by
Hubble's Advanced Camera for Surveys (ACS) and the Near Infrared
Camera and Multi-object Spectrometer (NICMOS). Both images reveal
galaxies that are too faint to be seen by ground-based telescopes,
or even in Hubble's previous faraway looks, called the Hubble Deep
Fields (HDFs), taken in 1995 and 1998.

Q & A: Understanding the Discovery

1.
How faint are the farthest objects?

The Hubble observations detected objects as faint as 30th
magnitude. The faintest objects the human eye can see are
at sixth magnitude. Ground-based telescopes also can
detect 30th-magnitude objects. Those objects, however, are
so dim they are lost in the glare of brighter, nearby
galaxies.

Searching for the faintest objects in the Ultra Deep Field is
like trying to find a firefly on the Moon. Light from the
farthest objects reached the Hubble telescope in trickles
rather than gushers. The orbiting observatory collected one
photon of light per minute from the dimmest objects.
Normally, the telescope collects millions of photons per
minute from nearby galaxies.

2.
How many orbits did it take to make the observations?

It took 400 orbits to make the observations.

3.
How many exposures were needed to make the observations?

The Hubble telescope's Advanced Camera for Surveys'
wide-field camera snapped 800 exposures, which equals
two exposures per orbit. The exposures were taken over
four months, from Sept. 24, 2003 to Jan. 16, 2004.

4.
How much viewing time was needed to make all the exposures?

The 800 exposures amounted to about 1 million seconds or
11.3 days of viewing time. The average exposure time was
21 minutes.

5.
How many galaxies are in the image?

The image yields a rich harvest of about 10,000 galaxies.

6.
How many colors (filters) were used to make the observations?

The colors used were blue, green, red, and near-infrared. The
observations were taken in visible to near-infrared light.

7.
If astronomers made the Hubble Ultra Deep Field observation over the entire sky, how long would it take?

The whole sky contains 12.7 million times more area than
the Ultra Deep Field. To observe the entire sky would take
almost 1 million years of uninterrupted observing.

8.
How wide is the Ultra Deep Field's slice of the heavens?

The Hubble Ultra Deep Field is called a "pencil beam"
survey because the observations encompass a narrow, yet
"deep" piece of sky. Astronomers compare the Ultra Deep
Field view to looking through an eight-foot-long soda
straw.

The Ultra Deep Field's patch of sky is so tiny it would fit
inside the largest impact basin that makes up the face on
the Moon. Astronomers would need about 50 Ultra Deep
Fields to cover the entire Moon.

9.
How sharp is Hubble's resolution in pinpointing far-flung galaxies in the Ultra Deep Field?

Hubble's keen vision (0.085 arc seconds.) is equivalent to standing at
the U.S. Capitol and seeing the date on a quarter a mile away at the
Washington monument.